Journal of Materials Research
November 2005— Volume 20, Number 11

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Shear strength of Sn-3.5Ag solder bumps formed on Ni/Au and organic solderability preservative surface-finished bond pads after multiple reflow steps Jung-Sub Lee, Kun-Mo Chu, and Duk Young Jeon Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-701, Republic of Korea

Sn-3.5Ag solder bumps were formed on electroless Ni/immersion Au (Ni/Au) and organic solderability preservative (OSP) surface-finished bond pads, respectively. The shear strength of the solder bumps was measured as a function of reflow steps. Fracture surfaces and interfacial microstructures were investigated by scanning electron microscope. The shear strength of Ni/Au samples increased up to the seventh reflow step and subsequently decreased after the tenth reflow step. Spalling of Ni₃Sn₄ intermetallic compounds (IMCs) and the P-rich Ni layer strengthened and weakened the bond, respectively. For OSP samples, although Cu₆Sn₅ IMCs grew as the reflow step was repeated, no remarkable change in shear strength was observed. Interfacial fractures of OSP samples occurred at the interface between Cu₆Sn₅ IMC and Cu₃Sn IMC. Fracture surfaces of OSP samples showed concave pits that consisted of a Cu₃Sn bottom and an Sn wall. The pits were formed by separation of Cu₆Sn₅ IMC from Cu₃Sn IMC and the molten Sn channel between the Cu₆Sn₅ IMC grains.

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